Cellular communication systems presently implemented in the U.S. utilize digitized voice and data signals for communication between a mobile communication device, generally a cellular telephone, and a base station. Movement of the mobile communication device at varying speeds (for instance if the user holding the device is walking, running, riding a bike, or in a car or train, etc.) causes channel fading to occur across the communication channels; the change in the signal caused by channel fading varying with time.
Prior art equalizers, which are included in the mobile communication device, generally include an estimator which receives the transmitted digitized signal and produces an output representing a distorted version of the digital signal as actually transmitted. A channel tracker is also included within the equalizer which receives both the transmitted digitized signal and the output of the equalizer, and produces a channel estimate signal representing an approximation of the channel fading that has occurred to the transmitted digitized signal. The role of the channel tracker is to use the output of the estimator to estimate the changes in the signal caused by channel fading, and to produce a channel estimate signal representing an approximation of the changes in the signal due to channel fading. The channel estimate signal produced by the channel tracker is utilized by the estimator to produce a more accurate representation of the digital signal as actually transmitted, i.e., get rid of the changes due to channel fading.
When the mobile communication device is moving at high speeds, the rate of change of the characteristics of a radio channel used by the mobile communication device typically occurs quickly. The opposite is true when the mobile communication device is moving at low speeds. When a channel tracker is built, an assumption must be made about the speed of the mobile communication device in which it is to be utilized. The channel tracker is tuned to the assumed speed and can accurately track the rate of change of the channel's characteristics if the mobile communication device is moving at a speed which approximates the speed to which the channel tracker is tuned. As a result, a channel tracker tuned to a high speed (HS) channel is designed such that it tracks fast changes in the channel's characteristics well. On the other hand, a channel tracker tuned to a low speed (LS) channel is designed such that it tracks slow changes in the channel's characteristics well.
When an HS channel tracker is used in an LS channel, it will tend to track non-existing changes in the channel's characteristics. This can hurt performance by about 1-2 dB. When an LS channel tracker is used in an HS channel, it may not be able to keep up with the changes in the channel's characteristics and can lose the channel completely by the end of a frame. For this reason, an HS tracker is typically chosen and a performance hit is taken at lower speeds.
However, merely placing an HS channel tracker in a wireless communication device or cellular phone is not quite satisfactory. Many people use their cellular phones essentially as wireless phones, that is, at extremely low speeds, e.g., 1-2 kilometers per hour, where the channel is almost static. The same is generally true for personal communicator phones and phones used as wireless modems for portable computers. In such cases, an HS channel tracker is a mismatch.
While there have been successful attempts to actually estimate the speed of the mobile communication device from the received signal, tracking the speed on top of tracking the channel is extremely complex and may result in a computational nightmare. Nevertheless, it is helpful to boost performance by using a channel tracker that is matched to the speed of the mobile communication device.
The present invention is directed toward overcoming one or more of the above-mentioned problems.